Toothbrush drive shaft and method for production thereof

ABSTRACT

The present invention is directed to a toothbrush drive shaft having a shaft shank mounting a force transmission piece, in particular an eccentric crank piece, in a manner preventing relative rotation. Furthermore, the present invention relates to a method of manufacturing such a drive shaft, whereby the force transmission piece and the shaft shank are separately produced and subsequently joined together. According to the invention, the force transmission piece is bent from a wire having a section thereof bent to form a helical wound body and to be pushed onto the shaft shank so it wraps around the shaft shank and sits thereon in a manner preventing relative rotation.

This invention relates to a drive shaft for small electric appliancesfor personal use. The present invention relates in particular to atoothbrush drive shaft with a shaft shank mounting a force transmissionpiece, in particular an eccentric crank piece, in a manner preventingrelative rotation. Furthermore, the present invention relates to amethod of manufacturing such a drive shaft, whereby the forcetransmission piece and the shaft shank are separately produced andsubsequently joined together.

In electric toothbrushes the brush head is as a rule connected with theelectric motor of the toothbrush by means of a transmission thatconverts the rotary drive motion of the electric motor into anoscillatory rotary drive motion, causing the brush head to be driven ina reciprocating motion. Typically, provision is made for a secondaryshaft coupled to the brush head and including an eccentric crank piecethrough which it is driven from the motor by a further part of the drivetrain or transmission.

A drive shaft of this type is disclosed, for example, in EP 0 560 758B1, whereby the shaft end close to the motor mounts a crank platecarrying an eccentric coupling pin arranged parallel to the longitudinalaxis of the shaft. This known drive shaft is, however, capable ofimprovement in various regards. In view of the tolerances to be held,the force transmission piece constructed as crank plate is difficult,and hence expensive, to manufacture. On the one hand, it is necessaryfor the coupling pin to be anchored on the crank plate in a preciselydefined position. The bore provided for this purpose in the crank platerequires correspondingly high manufacturing accuracy. On the other hand,also the bore with which the crank plate sits on the shaft shank has tobe manufactured to very small tolerances in order to obtain the desiredpress fit and to be able to transmit the necessary torques.

It is, therefore, an object of the present invention to provide animproved drive shaft of the type initially referred to and an improvedmethod for its production, which avoid the disadvantages of the priorart and develop it further in advantageous manner. Preferably, theobject is to provide an eccentric force transmission piece for the driveshaft that affords ease and economy of manufacture while yettransmitting moments reliably.

The object identified in the foregoing is accomplished with a driveshaft according to patent claim 1. With regard to the production method,the object identified is accomplished with a method according to patentclaim 10. Preferred embodiments of the invention are the subject-matterof the dependent claims.

The present invention hence provides for the force transmission piece topossess a wound body wrapping around the shaft shank in a mannerpreventing relative rotation. In lieu of a crank plate having a mountingbore for mounting on the shaft shank in a press-fit relationshipthereto, provision is made for a helically coiled fastening sectionsitting on the shaft shank by frictional engagement therewith.

In a further development of the present invention, the forcetransmission piece including its wound body and a function arm connectedtherewith may be integrally made of one piece, being in particularformed of bent wire. The force transmission piece may be made of springsteel spring so that it is elastic, forming a spring body.

As function arm provision may be made in particular for a freelyprojecting crank arm by means of which a moment is transmissible fromand to the shaft. Preferably, the crank arm is an eccentric coupling pinextending roughly parallel to the shaft longitudinal axis and suitablefor engagement by the next component in the gear train. In thisarrangement the coupling pin is preferably formed by a protruding end ofthe wire from which the wound body of the spring element is wound.

Advantageously, the spring body, meaning the wound body, may bemanufactured at very low cost on a conventional spring coiling machinefor compression and tension springs. With respect to processengineering, the invention hence provides for the force transmissionpiece to be bent from a length of wire a section of which is bent toform the helical wound body and is then pushed onto the shaft shank sothat the wound body sits on the shaft shank in non-rotatablerelationship thereto. In this process it is not necessarily the woundbody that has to be moved in order to be fitted onto the shaft shank. Itis also possible for the shaft to be pushed into the stationary andfixedly held wound body.

It will be appreciated that winding the wound body directly onto theshaft shank may also be contemplated. However, it is of particularadvantage to first manufacture the wound body, without the shaft beinginvolved, to an inside diameter smaller than the outside diameter of theshaft shank and then, using an expansion tool, to expand the wound bodyelastically to a mounting diameter which is at least as large as theoutside diameter of the shaft shank and slide it onto the shaft shank.As soon as the wound body occupies the desired position on the shaftshank, the elastic expansion is canceled by withdrawal of the expansiontool, causing the wound body to return or attempt to return to itsinitial diameter, whereby it wraps tightly around the shaft shank. As aresult, it is permanently held on the shaft shank by frictionalengagement therewith.

In order to achieve the desired rotational alignment of the forcetransmission piece relative to the shaft, use can be made of alignmentelements on the expansion tool, by means of which the function arm ofthe force transmission piece on the one hand and, on the other hand, thesection of the shaft shank defining the rotational orientation aresensed or gripped. Conventionally, the end of the shaft shank remotefrom the force transmission piece may possess a flattening or a notch soit is not completely symmetrical about its axis and it is necessary toprovide for the rotational alignment of the force transmission pieceduring assembly.

As an alternative or addition to the frictional seating engagement ofthe wound body with the shaft shank, provision could also be made forpositive engagement between the wound body and the shaft shank. Forexample, an oval cross-section of the shaft shank in the region of theforce transmission piece and a corresponding oval contour of the woundbody could be provided. Preferably, however, the wound body sits on thecircular shaft shank solely by frictional engagement therewith. In theevent of an overload condition occurring, this enables slipping of theforce transmission piece to be accomplished in advantageous manner,without damaging the connection between the force transmission piece andthe shaft shank. In contrast thereto, overloading invariably results inpermanent damage in cases where conventional crank plates are used. Inthe case of a conventional crank plate the sliding moment issubstantially lower than the loosening moment, rendering the assemblyuseless in the event of the connection being overloaded. By contrast, onoverloading and corresponding slipping of a force transmission piecewith the wound body of the invention, the corresponding assembly remainsfit for use. It is only necessary to perform once again a rotationalalignment between the wound body, meaning the force transmission piece,and the shaft shank. The loosening and the sliding moments of the woundbody sitting on the shaft shank by frictional engagement therewith arelargely identical.

Both the wound body and the shaft shank are suitably shaped in acircular cylindrical configuration. The shaft shank is circularcylindrical at least in that section thereof which mounts the woundbody.

The wrap angle of the wound body can be selected differently independence upon the torque to be transmitted. In accordance withEytelwein's principle, the transmissible torque can be increased withthe wrap angle increasing. In an advantageous aspect of the invention,the wrap angle is between 4 π and 12 π, preferably 6 π to 10 π. In viewof the typical torques to be transmitted by the drive mechanism ofelectric toothbrushes, the wrap angle may amount to about 8 π, that is,the helical wound body wraps around the shaft about four times.

The material thickness of the wire from which the force transmissionpiece is wound may be selected differently, in particular it may beadapted to the diameter of the shaft shank. Depending on theapplication, the wire diameter may be selected in accordance with thediameter of the coupling pin carried by the coupling plate to bereplaced. This makes it possible to use the drive shaft as replacementpart without the need to modify the remaining parts of the transmissionbetween electric motor and toothbrush head.

In a further development of the invention, the wire used for the woundbody has a material diameter smaller than the diameter of the shaftshank, amounting preferably to about ¼ to ½ of the shaft shank diameter.

The present invention will be described in more detail in the followingwith reference to a preferred embodiment and the accompanying drawings.In the drawings,

FIG. 1 is a sectional view of an electric toothbrush with a motor-drivensecondary shaft according to a preferred embodiment of the invention,whereby the secondary shaft mounts a crank piece functioning as forcetransmission piece and having a wound section made of spring steel;

FIG. 2 is an enlarged side view of the secondary shaft mounting thecrank piece of the toothbrush of FIG. 1;

FIG. 3 is a top plan view of the end of the secondary shaft of FIG. 2showing the rotational alignment of the crank piece relative to theflattened end of the secondary shaft;

FIG. 4 is an enlarged top plan view of the crank piece constructed asspring body with wound section from the preceding Figures, with thepartial view (b) being rotated through 90° relative to the partial view(a), and the partial view (c) being a section through the spring bodytaken along the line A-A of the partial view (a); and

FIG. 5 is a top plan view of the end of the spring body of FIG. 4.

The electric toothbrush illustrated in FIG. 1 possesses in a mannerknown per se a casing 1, which constitutes the handpiece of thetoothbrush and accommodates an electric driving mechanism for impartingmovement to a brush attachment not shown. Received in the casing 1 arean electric motor 2 and a transmission 3 driven thereby and comprising asecondary shaft 4 extending from the end of the casing 1 in order todrive a brush head attachable to the end of the casing 1 in anoscillatory rotary movement. For this purpose, the secondary shaft 4 hasat its forward end a flattening 5 and a notch provided opposite saidflattening to enable coupling engagement with the drive train receivedin the brush head. Provided at the opposite rear end of the casing 1 isa battery holder 7 receiving batteries for the supply of power to theelectric motor 2.

As shown in FIG. 1, the secondary shaft 4 and the electric motor 2 withits motor output shaft 8 are arranged parallel, yet offset to oneanother. Between the motor output shaft 8 and the secondary shaft 4 thetransmission 3 comprises various engagement and offsetting parts such ascrank, coupling and rocker, as well as finally the crank piece 9 sittingon the secondary shaft 4 in a manner prevent relative rotation, as willbe explained, in order to translate the rotary motion of the motoroutput shaft 8 into an oscillatory rotational motion of the secondaryshaft 4 in a manner known in the art.

The secondary shaft 4 is rotatably mounted in the casing 1, yet fixedlycarried therein axially. With its rear end it sits in a bearing bushing10 that absorbs axial thrust. The secondary shaft 4 may be securedagainst being pulled out by the crank piece 9 sitting fixedly on theshaft shank, through which crank piece the shaft abuts or would abutaxially a shoulder 11 formed fast with the casing if pulling forces acton the shaft.

As FIG. 2 shows, the secondary shaft 4 comprises a straight, elongateshaft shank 12 shaped in a cylindrical configuration of circularcross-section with the exception of its end providing the flattening 5and the notch 6. The shaft shank may be suitably made of steel or asimilarly high-strength material.

As FIGS. 2 and 3 show, the crank piece 9 comprises a helical wound body13 and a function arm 14 fixedly connected with the wound body 13 andshaped in the manner of a crank pin projecting from the shaft shank 12eccentrically. As FIG. 2 shows, the crank arm or crank pin extendsessentially parallel to the longitudinal axis of the shaft shank 12.

The wound body 13 and the crank arm 14 are integrally made of one piece,forming a spring body made of spring steel. The wound body 13 ishelically coiled in the manner of a cylinder spring. At one end thespring steel ends in the crank arm 14, which initially extends radiallybeyond the circumference of the wound body 13 and is finally angled inthe longitudinal direction of the wound body, causing the free end ofthe crank arm 14 to extend parallel to the longitudinal axis of thewound body 13 (see FIGS. 4 (a) and (b)). At the opposite end the springsteel, from which the crank piece 9 is wound, ends likewise slightlyradially, as FIG. 5 shows, which facilitates the expansion of the woundbody for the purpose of mounting it on the shaft shank, as will beexplained later. As FIG. 5 shows, the ends of the wire from which thecrank piece 9 is wound extend in different directions, including betweenthem an acute angle as seen in top plan view.

Advantageously, the crank piece 9 with its wound body 13 may bemanufactured at very low cost using a conventional spring coilingmachine for compression or tension springs. Different numbers of coilsof the wound body 13 may be selected and adapted to the torque to betransmitted. In the embodiment shown, provision is made for fourcomplete coils, that is, the wrap angle of the wound body 13 amounts toabout 8 π.

The wound body 13 is coiled on the spring coiling machine to an initialinside diameter that is smaller than the outside diameter of the shaftshank 12. By means of an expansion tool not shown in greater detail, thewound body 13 is then slightly expanded, enabling the shaft shank 12 tobe inserted into the wound body 13 in axial direction until it occupiesthe position shown in FIG. 2. As this occurs, care must be taken toensure that the crank piece 9 assumes the desired rotational alignmentrelative to the flattening 5. Advantageously, the expansion tool gripsthe crank arm 14 of the spring body. As the shaft shank 12 is beinginserted, the expansion tool at the same time senses or grips itsflattening 5 so that the crank arm 14 of the crank piece 9 invariablycomes to lie on the shaft shank in the desired alignment relative to theflattening 5.

The elastic expansion of the wound body 13 is then canceled by theexpansion tool being released or withdrawn. The wound body 13 will resetitself, contracting radially in its diameter so as to make frictionalengagement with the shaft shank 12, sitting thereon in a mannerpreventing relative rotation.

It will be understood that the crank arm 14 could be angled in differentdirections. Other than the illustration in the Figures, the freelyprojecting end forming the crank arm 14 could be bent back, meaning bentdownwardly according to FIG. 4, so that it would extend back over thewound body 13 and opposite the wound body and radially outside thereof.In the embodiment illustrated in FIG. 4, the crank arm 14 extendshowever in a direction away from the wound body 13 so that it projectsabove the end of the wound body 13 in axial direction. This has theadvantage that, while the lever arm is maintained unchanged, moreclearance remains in radial direction between the shaft shank and thecrank arm 14 because the wound body 13 does not extend therebetween.This enables the rocker of the transmission 3 to have more substance inthe area around the recess engaged by the crank arm 14.

Spring steel of different diameters may be used for the crank piece 9,and the material thickness may be adapted to the moments to betransmitted or also to the diameter of the shaft shank. Given thetypical diameters of toothbrush secondary shafts, the material thicknessof the spring steel from which the wound body 13 is coiled amounts tobetween 1 and 2 mm, preferably between 1 and 1.5 mm. In the embodimentillustrated, provision is made for a spring wire with a circulardiameter of 1.2 mm, approximately.

The secondary shaft shown in FIG. 2 is inserted into the casing 1 of theelectric toothbrush together with the crank piece 9 mounted thereonnon-rotatatively so that the secondary shaft is held in the bearingsprovided for this purpose. The crank piece 9 with its crank arm 14 isfitted into the receiving bore or, where applicable, longitudinal grooveof the rocker directly succeeding in the drive train, being hence inengagement therewith. The back-and-forth movement of the rocker is thentranslated by the crank piece 9 into an oscillatory rotary drivingmovement of the secondary shaft 4.

Apart from the embodiment described in the foregoing, the forcetransmission piece with the wound body sitting on the shaft in a mannerpreventing relative rotation may also find use as motor eccentric or astransverse axis on straight shafts. Preferably, however, the forcetransmission piece previously described constitutes the crank piece 9 ofthe secondary shaft of an electric toothbrush.

1-12. (canceled)
 13. An electric toothbrush comprising: a casing; anelectric motor enclosed in the casing, the electric motor comprising: amotor output shaft; a transmission driven by the motor output shaft, thetransmission comprising a crank piece that includes both a helicallywound body and a secondary shaft with a shaft shank defining an outerdiameter: the helically wound body disposed about the shaft shank andwound to have a body inner diameter smaller, in a relaxed state, thanthe outer diameter of the shaft shank, such that the wound body isfrictionally engaged against the secondary shaft for resisting relativerotational movement between the wound body and the secondary shaft, andthe secondary shaft extending outward from the wound body through adistal end of the casing; and a brush head operably connected to adistal end of the secondary shaft.
 14. The electric toothbrush of claim13, wherein the crank piece further comprises a function arm connectedwith the wound body so that rotational movement of the function arm,about a longitudinal axis of the shaft shank, applies a rotational forceto the secondary shaft through the wound body.
 15. The electric toothbrush of claim 14 wherein the wound body and function arm are integrallymade of one piece.
 16. The electric toothbrush of claim 15, wherein theone piece is formed of bent wire.
 17. The electric toothbrush of claim14, wherein the function arm comprises an eccentric coupling pinextending roughly parallel to the longitudinal axis of the shaft shank.18. The electric toothbrush of claim 17, wherein the coupling pinextends axially from, and extends beyond, the wound body.
 19. Theelectric toothbrush of claim 13, wherein the wound body is made ofspring steel.
 20. The electric toothbrush of claim 13, wherein the woundbody is held on the shaft shank primarily by the frictional engagementdue an elastic bias of the wound body.
 21. The electric toothbrush ofclaim 13, wherein the wound body and the shaft shank have matingcross-sectional contours at least where the wound body wraps around theshaft shank.
 22. The electric toothbrush of claim 21, wherein the matingcross-sectional contours of the wound body and the shaft shank are ofessentially circular cylindrical configuration and resistance torelative rotation between the wound body and the shaft shank isprimarily due to frictional engagement between the wound body and theshaft shank provided by an elastic bias of the wound body.
 23. Theelectric toothbrush of claim 21, wherein the mating cross-sectionalcontours are of substantially non-circular configuration so thatpositive engagement between the wound body and the shaft shank providesan additional resistance to relative rotation between the wound body andthe shaft shank.
 24. The electric toothbrush of claim 13, wherein a wrapangle of the wound body around the shaft shank is in the range fromabout 4 π to about 12 π.
 25. The electric toothbrush of claim 24,wherein the wrap angle of the wound body around the shaft shank is inthe range from about 6 π and about 10 π.
 26. The electric toothbrush ofclaim 25, wherein the wrap angle of the wound body around the shaftshank is in the range from about 7.5 π and about 8.5 π.
 27. The electrictoothbrush of claim 13, wherein the wound body has coils with a materialdiameter smaller than the shaft shank diameter.
 28. The electrictoothbrush of claim 27, wherein the material diameter of the coilsranges between about one-quarter to about one-half of the shaft shankdiameter.
 29. A transmission for an electric toothbrush driven by amotor output shaft of an electric motor, the transmission comprising: acrank piece that includes a helically wound body with a natural shapedefining a body inner diameter, and a secondary shaft that extendsoutward from the wound body to a brush head drive train; wherein thebody inner diameter is smaller than a shaft outer diameter of thesecondary shaft and the wound body is mounted around a shaft shank ofthe secondary shaft so that an elastic bias of the wound body towardsresuming its natural shape brings the wound body and the shaft shankinto a frictional engagement that resists relative rotational movementbetween the wound body and the shaft shank.
 30. The transmission ofclaim 29, wherein the crank piece further comprises a function armconnected with the wound body so that rotational movement of thefunction arm, about a longitudinal axis of the shaft shank, applies arotational force to the secondary shaft through the wound body.
 31. Thetransmission of claim 30 wherein the wound body and function arm areintegrally made of one piece.
 32. The transmission of claim 31, whereinthe one piece is formed of bent wire.
 33. The transmission of claim 30,wherein the function arm comprises an eccentric coupling pin extendingroughly parallel to the longitudinal axis of the shaft shank.
 34. Thetransmission of claim 33, wherein the coupling pin extends axially fromthe wound body so it protrudes beyond the end of the wound body.
 35. Thetransmission of claim 29, wherein the wound body is made of springsteel.
 36. The transmission of claim 29, wherein the wound body is heldon the shaft shank primarily by the frictional engagement due theelastic bias of the wound body.
 37. The transmission of claim 29,wherein the wound body and the shaft shank have mating cross-sectionalcontours at least where the wound body wraps around shaft shank.
 38. Thetransmission of claim 37, wherein the mating cross-sectional contours ofthe wound body and the shaft shank are of essentially circularcylindrical configuration and the resistance to relative rotationbetween the wound body and the shaft shank is solely due to primarilybased on frictional engagement of between the wound body and the shaftshank provided by the elastic bias of the wound body.
 39. Thetransmission of claim 37, wherein the mating cross-sectional contoursare of substantially non-circular configuration so that positiveengagement between the wound body and the shaft shank provides anadditional resistance to relative rotation between the wound body andthe shaft shank.
 40. The transmission of claim 29, wherein a wrap angleof the wound body around the shaft shank is in the range from about 4 πto about 12 π.
 41. The transmission of claim 40, wherein the wrap angleof the wound body around the shaft shank is in the range from about 6 πand about 10 π.
 42. The transmission of claim 41, wherein the wrap angleof the wound body around the shaft shank is in the range from about 7.5π and about 8.5 π.
 43. The transmission of claim 29, wherein the woundbody has coils with a material diameter smaller than a shaft shankdiameter.
 44. The transmission of claim 43, wherein the materialdiameter of the coils ranges between about one-quarter to about one-halfof the shaft shank diameter.
 45. A drive shaft for small electricappliances, the drive shaft comprising a shaft shank mounting a forcetransmission piece, wherein the force transmission piece includes awound body wrapping around the shaft shank in a manner resistingrelative rotation between the wound body and the shaft shank.
 46. Thedrive shaft of claim 45 wherein the force transmission piece alsoincludes a function arm connected with the wound body.
 47. The driveshaft of claim 46 wherein the wound body and the function arm areintegrally made of one piece.
 48. The drive shaft of claim 46 whereinthe function arm has an eccentric coupling pin extending roughlyparallel preferably to the shaft shank longitudinal axis.
 49. The driveshaft of claim 48 wherein the coupling pin extends axially from thewound body so it protrudes beyond the latter's end.
 50. The drive shaftof claim 47 wherein the one piece is formed of bent wire.
 51. The driveshaft of claim 45 wherein the force transmission piece forms a springbody.
 52. The drive shaft of claim 49 wherein the force transmissionpiece is made of spring steel.
 53. The drive shaft of claim 45 whereinthe wound body is held on the shaft shank under elastic bias solely byfrictional engagement therewith.
 54. The drive shaft of claim 45 whereinthe wound body and the shaft shank have mating cross-sectional contoursat least in that shank section in which the wound body wraps around theshaft shank.
 55. The drive shaft of claim 54 wherein the matingcross-sectional contours are of circular cylindrical configuration. 56.The drive shaft of claim 45 wherein a wrap angle of the wound bodyaround the shaft shank is in the range from about 4 π to about 12 π. 57.The drive shaft of claim 56 wherein the wrap angle is in the rangebetween about 6 π and about 10 π.
 58. The drive shaft of claim 56wherein the wrap angle is in the range between about 7.5 π and about 8.5π.
 59. The drive shaft of claim 45 wherein the wound body has coils witha material diameter smaller than a shaft shank diameter.
 60. The driveshaft of claim 59 wherein the material diameter is in the range betweenabout one-quarter to one-half of the shaft shank diameter.